NCERT Textbook - Sensory, Attentional and Perceptual Processes Humanities/Arts Notes | EduRev

Psychology Class 11

Humanities/Arts : NCERT Textbook - Sensory, Attentional and Perceptual Processes Humanities/Arts Notes | EduRev

 Page 1


Psychology
84
Chapter
5
• understand the nature of sensory processes,
• explain the processes and types of attention,
• analyse the problems of form and space perception,
• examine the role of socio-cultural factors in perception, and
• reflect on sensory, attentional and perceptual processes in everyday life.
After reading this chapter, you would be able to
Sensory , Attentional and
Perceptual Processes
Sensory , Attentional and
Perceptual Processes
Introduction
Knowing the World
Nature and Varieties of Stimulus
Sense Modalities
Visual Sensation
Other Human Senses (Box 5.1)
Auditory Sensation
Attentional Processes
Selective Attention
Divided Attention (Box 5.2)
Sustained Attention
Span of Attention (Box 5.3)
Attention Deficit Hyperactivity Disorder (Box 5.4)
Perceptual Processes
Processing Approaches in Perception
The Perceiver
Principles of Perceptual Organisation
Perception of Space, Depth, and Distance
Monocular Cues and Binocular Cues
Perceptual Constancies
Illusions
Socio-Cultural Influences on Perception
Key Terms
Summary
Review Questions
Project Ideas
Contents
The quality of life is determined
by its activities.
– Aristotle
2019-20
Page 2


Psychology
84
Chapter
5
• understand the nature of sensory processes,
• explain the processes and types of attention,
• analyse the problems of form and space perception,
• examine the role of socio-cultural factors in perception, and
• reflect on sensory, attentional and perceptual processes in everyday life.
After reading this chapter, you would be able to
Sensory , Attentional and
Perceptual Processes
Sensory , Attentional and
Perceptual Processes
Introduction
Knowing the World
Nature and Varieties of Stimulus
Sense Modalities
Visual Sensation
Other Human Senses (Box 5.1)
Auditory Sensation
Attentional Processes
Selective Attention
Divided Attention (Box 5.2)
Sustained Attention
Span of Attention (Box 5.3)
Attention Deficit Hyperactivity Disorder (Box 5.4)
Perceptual Processes
Processing Approaches in Perception
The Perceiver
Principles of Perceptual Organisation
Perception of Space, Depth, and Distance
Monocular Cues and Binocular Cues
Perceptual Constancies
Illusions
Socio-Cultural Influences on Perception
Key Terms
Summary
Review Questions
Project Ideas
Contents
The quality of life is determined
by its activities.
– Aristotle
2019-20
Chapter 5 • Sensory, Attentional and Perceptual Processes
85
not only from the external world, but also from
our own body. The information collected by
our sense organs forms the basis of all our
knowledge. The sense organs register several
kinds of information about various objects.
However, in order to be registered, the objects
and their qualities (e.g., size, shape, colour)
must be able to draw our attention. The
registered information must also be sent to
the brain that constructs some meaning out
of them. Thus, our knowledge of the world
around us depends on three basic processes,
called sensation, attention, and perception.
These processes are highly interrelated; hence,
they are often considered as different elements
of the same process, called cognition.
NATURE AND VARIETIES OF STIMULUS
The external environment that surrounds us
contains a wide variety of stimuli. Some of
them can be seen (e.g., a house), while some
can be heard only (e.g., music). There are
several others that we can smell (e.g., fragrance
of a flower) or taste (e.g., sweets). There are
still others that we can experience by touching
(e.g., softness of a cloth). All these stimuli
KNOWING THE WORLD
The world in which we live is full of variety of
objects, people, and events. Look at the room
you are sitting in. You will find so many things
around. Just to mention a few, you may see
your table, your chair, your books, your bag,
your watch, pictures on the wall and many
other things. Their sizes, shapes, and colours
are also different. If you move to other rooms
of your house, you will notice several other
new things (e.g., pots and pans, almirah, TV).
If you go beyond your house, you will find still
many more things that you generally know
about (trees, animals, buildings). Such
experiences are very common in our day-to-
day life. We hardly have to make any efforts
to know them.
If someone asks you, “How can you say
that these various things exist in your room,
or house, or in the outside environment?”, you
will most probably answer that you see or
experience them all around you. In doing so,
you are trying to tell the person that the
knowledge about various objects becomes
possible with the help of our sense organs (e.g.,
eyes, ears). These organs collect information
In the previous chapters you have already learnt how we respond to various stimuli
present in the external and internal environment with the help of our receptors.
While some of these receptors are clearly observable (for example, eyes or ears),
others lie inside our body, and are not observable without the help of electrical or
mechanical devices. This chapter will introduce you to various receptors that collect
a variety of information from the external and internal worlds. The focus will be
particularly on the structure and function of eye and ear , including some interesting
processes associated with vision and audition. You will also know some important
things about attention, which helps us to notice and register the information that
our sense organs carry to us. Different types of attention will be described along
with the factors that influence them. At the end, we will discuss the process of
perception that allows us to understand the world in a meaningful way. You will
also have an opportunity to know how we are sometimes deceived by certain types
of stimuli such as figures and pictures.
Introduction
2019-20
Page 3


Psychology
84
Chapter
5
• understand the nature of sensory processes,
• explain the processes and types of attention,
• analyse the problems of form and space perception,
• examine the role of socio-cultural factors in perception, and
• reflect on sensory, attentional and perceptual processes in everyday life.
After reading this chapter, you would be able to
Sensory , Attentional and
Perceptual Processes
Sensory , Attentional and
Perceptual Processes
Introduction
Knowing the World
Nature and Varieties of Stimulus
Sense Modalities
Visual Sensation
Other Human Senses (Box 5.1)
Auditory Sensation
Attentional Processes
Selective Attention
Divided Attention (Box 5.2)
Sustained Attention
Span of Attention (Box 5.3)
Attention Deficit Hyperactivity Disorder (Box 5.4)
Perceptual Processes
Processing Approaches in Perception
The Perceiver
Principles of Perceptual Organisation
Perception of Space, Depth, and Distance
Monocular Cues and Binocular Cues
Perceptual Constancies
Illusions
Socio-Cultural Influences on Perception
Key Terms
Summary
Review Questions
Project Ideas
Contents
The quality of life is determined
by its activities.
– Aristotle
2019-20
Chapter 5 • Sensory, Attentional and Perceptual Processes
85
not only from the external world, but also from
our own body. The information collected by
our sense organs forms the basis of all our
knowledge. The sense organs register several
kinds of information about various objects.
However, in order to be registered, the objects
and their qualities (e.g., size, shape, colour)
must be able to draw our attention. The
registered information must also be sent to
the brain that constructs some meaning out
of them. Thus, our knowledge of the world
around us depends on three basic processes,
called sensation, attention, and perception.
These processes are highly interrelated; hence,
they are often considered as different elements
of the same process, called cognition.
NATURE AND VARIETIES OF STIMULUS
The external environment that surrounds us
contains a wide variety of stimuli. Some of
them can be seen (e.g., a house), while some
can be heard only (e.g., music). There are
several others that we can smell (e.g., fragrance
of a flower) or taste (e.g., sweets). There are
still others that we can experience by touching
(e.g., softness of a cloth). All these stimuli
KNOWING THE WORLD
The world in which we live is full of variety of
objects, people, and events. Look at the room
you are sitting in. You will find so many things
around. Just to mention a few, you may see
your table, your chair, your books, your bag,
your watch, pictures on the wall and many
other things. Their sizes, shapes, and colours
are also different. If you move to other rooms
of your house, you will notice several other
new things (e.g., pots and pans, almirah, TV).
If you go beyond your house, you will find still
many more things that you generally know
about (trees, animals, buildings). Such
experiences are very common in our day-to-
day life. We hardly have to make any efforts
to know them.
If someone asks you, “How can you say
that these various things exist in your room,
or house, or in the outside environment?”, you
will most probably answer that you see or
experience them all around you. In doing so,
you are trying to tell the person that the
knowledge about various objects becomes
possible with the help of our sense organs (e.g.,
eyes, ears). These organs collect information
In the previous chapters you have already learnt how we respond to various stimuli
present in the external and internal environment with the help of our receptors.
While some of these receptors are clearly observable (for example, eyes or ears),
others lie inside our body, and are not observable without the help of electrical or
mechanical devices. This chapter will introduce you to various receptors that collect
a variety of information from the external and internal worlds. The focus will be
particularly on the structure and function of eye and ear , including some interesting
processes associated with vision and audition. You will also know some important
things about attention, which helps us to notice and register the information that
our sense organs carry to us. Different types of attention will be described along
with the factors that influence them. At the end, we will discuss the process of
perception that allows us to understand the world in a meaningful way. You will
also have an opportunity to know how we are sometimes deceived by certain types
of stimuli such as figures and pictures.
Introduction
2019-20
Psychology
86
provide us with various kinds of information.
We have very specialised sense organs to deal
with these different stimuli. As human beings
we are bestowed with a set of seven sense
organs. These sense organs are also known
as sensory receptors or information gathering
systems, because they receive or gather
information from a variety of sources. Five of
these sense organs collect information from
the external world. These are eyes, ears, nose,
tongue, and skin. While our eyes are primarily
responsible for vision, ears for hearing, nose
for smell, and tongue for taste, skin is
responsible for the experiences of touch,
warmth, cold, and pain. Specialised receptors
of warmth, cold, and pain are found inside
our skin. Besides these five external sense
organs, we have also got two deep senses. They
are called  kinesthetic and vestibular systems.
They provide us with important information
about our body position and movement of
body parts related to each other. With these
seven sense organs, we register ten different
variety of stimuli. For example, you may notice
whether a light is bright or dim, whether it is
yellow, red or green, and so on. With sound
you may notice whether it is loud or faint,
whether it is melodious or distracting, and so
on. These different qualities of stimuli are also
registered by our sense organs.
SENSE MODALITIES
Our sense organs provide us with first-hand
information about our external or internal
world. The initial experience of a stimulus or
an object registered by a particular sense
organ is called sensation. It is a process
through which we detect and encode a variety
of physical stimuli. Sensation also refers to
immediate basic experiences of stimulus
attributes, such as “hard”, “warm”, “loud”, and
“blue”, which result from appropriate
stimulation of a sensory organ. Different sense
organs deal with different forms of stimuli and
serve different purposes. Each sense organ is
highly specialised for dealing with a particular
kind of information. Hence, each one of them
is known as a sense modality.
Functional Limitations of Sense Organs
Before we move on to a discussion of sense
organs, it is important to note that our sense
organs function with certain limitations. For
example, our eyes cannot see things which
are very dim or very bright. Similarly our ears
cannot hear very faint or very loud sounds.
The same is true for other sense organs also.
As human beings, we function within a limited
range of stimulation. For being noticed by a
sensory receptor, a stimulus has to be of an
optimal intensity or magnitude. The
relationship between stimuli and the
sensations they evoke has been studied in a
discipline, called psychophysics.
In order to be noticed a stimulus has to
carry a minimum value or weight. The
minimum value of a stimulus required to
activate a given sensory system is called
absolute threshold or absolute limen (AL).
For example, if you add a granule of sugar to
a glass of water, you may not experience any
sweetness in that water. Addition of a second
granule to water may also not make it taste
sweet. But if you go on adding sugar granules
one after another, there will come a point when
you will say that the water is now sweet. The
minimum number of sugar granules required
to say that the water is sweet will be the AL of
sweetness.
It may be noted at this point that the AL is
not a fixed point; instead it varies considerably
across individuals and situations depending
on the people’s organic conditions and their
motivational states. Hence, we have to assess
it on the basis of a number of trials. The
number of sugar granules that may produce
the experience of “sweetness” in water on
50 per cent of occasions will be called the AL
of sweetness. If you add more number of sugar
granules, the chances are greater that the
water will be reported more often as sweet than
plain.
As it is not possible for us to notice all
stimuli, it is also not possible to differentiate
between all stimuli. In order to notice two
stimuli as different from each other, there has
to be some minimum difference between the
2019-20
Page 4


Psychology
84
Chapter
5
• understand the nature of sensory processes,
• explain the processes and types of attention,
• analyse the problems of form and space perception,
• examine the role of socio-cultural factors in perception, and
• reflect on sensory, attentional and perceptual processes in everyday life.
After reading this chapter, you would be able to
Sensory , Attentional and
Perceptual Processes
Sensory , Attentional and
Perceptual Processes
Introduction
Knowing the World
Nature and Varieties of Stimulus
Sense Modalities
Visual Sensation
Other Human Senses (Box 5.1)
Auditory Sensation
Attentional Processes
Selective Attention
Divided Attention (Box 5.2)
Sustained Attention
Span of Attention (Box 5.3)
Attention Deficit Hyperactivity Disorder (Box 5.4)
Perceptual Processes
Processing Approaches in Perception
The Perceiver
Principles of Perceptual Organisation
Perception of Space, Depth, and Distance
Monocular Cues and Binocular Cues
Perceptual Constancies
Illusions
Socio-Cultural Influences on Perception
Key Terms
Summary
Review Questions
Project Ideas
Contents
The quality of life is determined
by its activities.
– Aristotle
2019-20
Chapter 5 • Sensory, Attentional and Perceptual Processes
85
not only from the external world, but also from
our own body. The information collected by
our sense organs forms the basis of all our
knowledge. The sense organs register several
kinds of information about various objects.
However, in order to be registered, the objects
and their qualities (e.g., size, shape, colour)
must be able to draw our attention. The
registered information must also be sent to
the brain that constructs some meaning out
of them. Thus, our knowledge of the world
around us depends on three basic processes,
called sensation, attention, and perception.
These processes are highly interrelated; hence,
they are often considered as different elements
of the same process, called cognition.
NATURE AND VARIETIES OF STIMULUS
The external environment that surrounds us
contains a wide variety of stimuli. Some of
them can be seen (e.g., a house), while some
can be heard only (e.g., music). There are
several others that we can smell (e.g., fragrance
of a flower) or taste (e.g., sweets). There are
still others that we can experience by touching
(e.g., softness of a cloth). All these stimuli
KNOWING THE WORLD
The world in which we live is full of variety of
objects, people, and events. Look at the room
you are sitting in. You will find so many things
around. Just to mention a few, you may see
your table, your chair, your books, your bag,
your watch, pictures on the wall and many
other things. Their sizes, shapes, and colours
are also different. If you move to other rooms
of your house, you will notice several other
new things (e.g., pots and pans, almirah, TV).
If you go beyond your house, you will find still
many more things that you generally know
about (trees, animals, buildings). Such
experiences are very common in our day-to-
day life. We hardly have to make any efforts
to know them.
If someone asks you, “How can you say
that these various things exist in your room,
or house, or in the outside environment?”, you
will most probably answer that you see or
experience them all around you. In doing so,
you are trying to tell the person that the
knowledge about various objects becomes
possible with the help of our sense organs (e.g.,
eyes, ears). These organs collect information
In the previous chapters you have already learnt how we respond to various stimuli
present in the external and internal environment with the help of our receptors.
While some of these receptors are clearly observable (for example, eyes or ears),
others lie inside our body, and are not observable without the help of electrical or
mechanical devices. This chapter will introduce you to various receptors that collect
a variety of information from the external and internal worlds. The focus will be
particularly on the structure and function of eye and ear , including some interesting
processes associated with vision and audition. You will also know some important
things about attention, which helps us to notice and register the information that
our sense organs carry to us. Different types of attention will be described along
with the factors that influence them. At the end, we will discuss the process of
perception that allows us to understand the world in a meaningful way. You will
also have an opportunity to know how we are sometimes deceived by certain types
of stimuli such as figures and pictures.
Introduction
2019-20
Psychology
86
provide us with various kinds of information.
We have very specialised sense organs to deal
with these different stimuli. As human beings
we are bestowed with a set of seven sense
organs. These sense organs are also known
as sensory receptors or information gathering
systems, because they receive or gather
information from a variety of sources. Five of
these sense organs collect information from
the external world. These are eyes, ears, nose,
tongue, and skin. While our eyes are primarily
responsible for vision, ears for hearing, nose
for smell, and tongue for taste, skin is
responsible for the experiences of touch,
warmth, cold, and pain. Specialised receptors
of warmth, cold, and pain are found inside
our skin. Besides these five external sense
organs, we have also got two deep senses. They
are called  kinesthetic and vestibular systems.
They provide us with important information
about our body position and movement of
body parts related to each other. With these
seven sense organs, we register ten different
variety of stimuli. For example, you may notice
whether a light is bright or dim, whether it is
yellow, red or green, and so on. With sound
you may notice whether it is loud or faint,
whether it is melodious or distracting, and so
on. These different qualities of stimuli are also
registered by our sense organs.
SENSE MODALITIES
Our sense organs provide us with first-hand
information about our external or internal
world. The initial experience of a stimulus or
an object registered by a particular sense
organ is called sensation. It is a process
through which we detect and encode a variety
of physical stimuli. Sensation also refers to
immediate basic experiences of stimulus
attributes, such as “hard”, “warm”, “loud”, and
“blue”, which result from appropriate
stimulation of a sensory organ. Different sense
organs deal with different forms of stimuli and
serve different purposes. Each sense organ is
highly specialised for dealing with a particular
kind of information. Hence, each one of them
is known as a sense modality.
Functional Limitations of Sense Organs
Before we move on to a discussion of sense
organs, it is important to note that our sense
organs function with certain limitations. For
example, our eyes cannot see things which
are very dim or very bright. Similarly our ears
cannot hear very faint or very loud sounds.
The same is true for other sense organs also.
As human beings, we function within a limited
range of stimulation. For being noticed by a
sensory receptor, a stimulus has to be of an
optimal intensity or magnitude. The
relationship between stimuli and the
sensations they evoke has been studied in a
discipline, called psychophysics.
In order to be noticed a stimulus has to
carry a minimum value or weight. The
minimum value of a stimulus required to
activate a given sensory system is called
absolute threshold or absolute limen (AL).
For example, if you add a granule of sugar to
a glass of water, you may not experience any
sweetness in that water. Addition of a second
granule to water may also not make it taste
sweet. But if you go on adding sugar granules
one after another, there will come a point when
you will say that the water is now sweet. The
minimum number of sugar granules required
to say that the water is sweet will be the AL of
sweetness.
It may be noted at this point that the AL is
not a fixed point; instead it varies considerably
across individuals and situations depending
on the people’s organic conditions and their
motivational states. Hence, we have to assess
it on the basis of a number of trials. The
number of sugar granules that may produce
the experience of “sweetness” in water on
50 per cent of occasions will be called the AL
of sweetness. If you add more number of sugar
granules, the chances are greater that the
water will be reported more often as sweet than
plain.
As it is not possible for us to notice all
stimuli, it is also not possible to differentiate
between all stimuli. In order to notice two
stimuli as different from each other, there has
to be some minimum difference between the
2019-20
Chapter 5 • Sensory, Attentional and Perceptual Processes
87
value of  those stimuli. The smallest difference
in the value of two stimuli that is necessary to
notice them as different is called difference
threshold or difference limen (DL). To
understand it, we may continue with our
“sugar water” experiment. As we have seen,
the plain water is experienced as sweet after
the addition of certain number of sugar
granules. Let us remember this sweetness. The
next question is: how many sugar granules
will be needed in the water in order to
experience its sweetness as different from the
previous sweetness. Go on adding sugar
granules one after another tasting the water
each time. After addition of a few granules,
you will notice at a point that the water is now
sweeter than the previous one. The number
of sugar granules added to the water to
generate an experience of sweetness that is
different from the previous sweetness on 50
per cent of the occasions will be called the DL
of sweetness. Thus, difference threshold is the
minimum amount of change in a physical
stimulus that is capable of producing a
sensation difference on 50 per cent of the
trials.
You may realise by now that understanding
of sensations is not possible without
understanding the AL and DL of different types
of stimuli (for example, visual, auditory), but
that is not enough. Sensory processes do not
depend only on the stimulus characteristics.
Sense organs and the neural pathways
connecting them to various brain centers also
play a vital role in this process. A sense organ
receives the stimulus and encodes it as an
electrical impulse. For being noticed this
electrical impulse must reach the higher brain
centers. Any structural or functional defect or
damage in the receptor organ, its neural
pathway, or the concerned brain area may lead
to a partial or complete loss of sensation.
Visual Sensation
Among all sense modalities, vision is the most
highly developed in human beings. Various
estimates indicate that we use it in
approximately 80 per cent of our transactions
with the external world. Audition and other
senses also contribute significantly to
information gathering from the external world.
We shall discuss vision and audition in some
detail. The main features of other senses can
be found in Box 5.1.
Visual sensation starts when light enters
the eyes and stimulates our visual receptors.
Our eyes are sensitive to a spectrum of light,
the wavelength of which ranges from 380 nm
to 780 nm (nm refers to nanometer, which is
one billionth of a meter). No sensation is
registered beyond this range of light.
The Human Eye
A diagram of the human eye is shown in
Figure 5.1. As you can see, our eye is made
up of three layers. In the outer layer, there is
a transparent cornea and a tough sclera that
surrounds the rest of the eye. It protects the
eye and maintains its shape. The middle layer
is called choroid, which is richly supplied with
blood vessels. The inner layer is known as
retina. It contains the photoreceptors (rods
and cones) and an elaborate network of
interconnecting neurons.
The eye is generally compared with a
camera. For example, the eye and camera have
a lens. The lens divides the eye into two
unequal chambers, namely aqueous chamber
and vitreous chamber. The aqueous chamber
is located between the cornea and the lens. It
is smaller in size and is filled with a water-
like substance, called aqueous humor. The
vitreous chamber is located between the lens
and the retina. It is filled with a jelly like
protein, called vitreous humor. These fluids
help in holding the lens at its appropriate place
and in proper shape. They also allow enough
flexibility for the occurrence of accommodation
— a process through which the lens changes
its shape in order to focus the objects at
varying distances. This process is regulated
by ciliary muscles, which are attached to the
lens. These muscles flatten the lens to focus
the distant objects and thicken it to focus the
near objects. Like a camera, the eye also has
a mechanism to control the amount of light
2019-20
Page 5


Psychology
84
Chapter
5
• understand the nature of sensory processes,
• explain the processes and types of attention,
• analyse the problems of form and space perception,
• examine the role of socio-cultural factors in perception, and
• reflect on sensory, attentional and perceptual processes in everyday life.
After reading this chapter, you would be able to
Sensory , Attentional and
Perceptual Processes
Sensory , Attentional and
Perceptual Processes
Introduction
Knowing the World
Nature and Varieties of Stimulus
Sense Modalities
Visual Sensation
Other Human Senses (Box 5.1)
Auditory Sensation
Attentional Processes
Selective Attention
Divided Attention (Box 5.2)
Sustained Attention
Span of Attention (Box 5.3)
Attention Deficit Hyperactivity Disorder (Box 5.4)
Perceptual Processes
Processing Approaches in Perception
The Perceiver
Principles of Perceptual Organisation
Perception of Space, Depth, and Distance
Monocular Cues and Binocular Cues
Perceptual Constancies
Illusions
Socio-Cultural Influences on Perception
Key Terms
Summary
Review Questions
Project Ideas
Contents
The quality of life is determined
by its activities.
– Aristotle
2019-20
Chapter 5 • Sensory, Attentional and Perceptual Processes
85
not only from the external world, but also from
our own body. The information collected by
our sense organs forms the basis of all our
knowledge. The sense organs register several
kinds of information about various objects.
However, in order to be registered, the objects
and their qualities (e.g., size, shape, colour)
must be able to draw our attention. The
registered information must also be sent to
the brain that constructs some meaning out
of them. Thus, our knowledge of the world
around us depends on three basic processes,
called sensation, attention, and perception.
These processes are highly interrelated; hence,
they are often considered as different elements
of the same process, called cognition.
NATURE AND VARIETIES OF STIMULUS
The external environment that surrounds us
contains a wide variety of stimuli. Some of
them can be seen (e.g., a house), while some
can be heard only (e.g., music). There are
several others that we can smell (e.g., fragrance
of a flower) or taste (e.g., sweets). There are
still others that we can experience by touching
(e.g., softness of a cloth). All these stimuli
KNOWING THE WORLD
The world in which we live is full of variety of
objects, people, and events. Look at the room
you are sitting in. You will find so many things
around. Just to mention a few, you may see
your table, your chair, your books, your bag,
your watch, pictures on the wall and many
other things. Their sizes, shapes, and colours
are also different. If you move to other rooms
of your house, you will notice several other
new things (e.g., pots and pans, almirah, TV).
If you go beyond your house, you will find still
many more things that you generally know
about (trees, animals, buildings). Such
experiences are very common in our day-to-
day life. We hardly have to make any efforts
to know them.
If someone asks you, “How can you say
that these various things exist in your room,
or house, or in the outside environment?”, you
will most probably answer that you see or
experience them all around you. In doing so,
you are trying to tell the person that the
knowledge about various objects becomes
possible with the help of our sense organs (e.g.,
eyes, ears). These organs collect information
In the previous chapters you have already learnt how we respond to various stimuli
present in the external and internal environment with the help of our receptors.
While some of these receptors are clearly observable (for example, eyes or ears),
others lie inside our body, and are not observable without the help of electrical or
mechanical devices. This chapter will introduce you to various receptors that collect
a variety of information from the external and internal worlds. The focus will be
particularly on the structure and function of eye and ear , including some interesting
processes associated with vision and audition. You will also know some important
things about attention, which helps us to notice and register the information that
our sense organs carry to us. Different types of attention will be described along
with the factors that influence them. At the end, we will discuss the process of
perception that allows us to understand the world in a meaningful way. You will
also have an opportunity to know how we are sometimes deceived by certain types
of stimuli such as figures and pictures.
Introduction
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Psychology
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provide us with various kinds of information.
We have very specialised sense organs to deal
with these different stimuli. As human beings
we are bestowed with a set of seven sense
organs. These sense organs are also known
as sensory receptors or information gathering
systems, because they receive or gather
information from a variety of sources. Five of
these sense organs collect information from
the external world. These are eyes, ears, nose,
tongue, and skin. While our eyes are primarily
responsible for vision, ears for hearing, nose
for smell, and tongue for taste, skin is
responsible for the experiences of touch,
warmth, cold, and pain. Specialised receptors
of warmth, cold, and pain are found inside
our skin. Besides these five external sense
organs, we have also got two deep senses. They
are called  kinesthetic and vestibular systems.
They provide us with important information
about our body position and movement of
body parts related to each other. With these
seven sense organs, we register ten different
variety of stimuli. For example, you may notice
whether a light is bright or dim, whether it is
yellow, red or green, and so on. With sound
you may notice whether it is loud or faint,
whether it is melodious or distracting, and so
on. These different qualities of stimuli are also
registered by our sense organs.
SENSE MODALITIES
Our sense organs provide us with first-hand
information about our external or internal
world. The initial experience of a stimulus or
an object registered by a particular sense
organ is called sensation. It is a process
through which we detect and encode a variety
of physical stimuli. Sensation also refers to
immediate basic experiences of stimulus
attributes, such as “hard”, “warm”, “loud”, and
“blue”, which result from appropriate
stimulation of a sensory organ. Different sense
organs deal with different forms of stimuli and
serve different purposes. Each sense organ is
highly specialised for dealing with a particular
kind of information. Hence, each one of them
is known as a sense modality.
Functional Limitations of Sense Organs
Before we move on to a discussion of sense
organs, it is important to note that our sense
organs function with certain limitations. For
example, our eyes cannot see things which
are very dim or very bright. Similarly our ears
cannot hear very faint or very loud sounds.
The same is true for other sense organs also.
As human beings, we function within a limited
range of stimulation. For being noticed by a
sensory receptor, a stimulus has to be of an
optimal intensity or magnitude. The
relationship between stimuli and the
sensations they evoke has been studied in a
discipline, called psychophysics.
In order to be noticed a stimulus has to
carry a minimum value or weight. The
minimum value of a stimulus required to
activate a given sensory system is called
absolute threshold or absolute limen (AL).
For example, if you add a granule of sugar to
a glass of water, you may not experience any
sweetness in that water. Addition of a second
granule to water may also not make it taste
sweet. But if you go on adding sugar granules
one after another, there will come a point when
you will say that the water is now sweet. The
minimum number of sugar granules required
to say that the water is sweet will be the AL of
sweetness.
It may be noted at this point that the AL is
not a fixed point; instead it varies considerably
across individuals and situations depending
on the people’s organic conditions and their
motivational states. Hence, we have to assess
it on the basis of a number of trials. The
number of sugar granules that may produce
the experience of “sweetness” in water on
50 per cent of occasions will be called the AL
of sweetness. If you add more number of sugar
granules, the chances are greater that the
water will be reported more often as sweet than
plain.
As it is not possible for us to notice all
stimuli, it is also not possible to differentiate
between all stimuli. In order to notice two
stimuli as different from each other, there has
to be some minimum difference between the
2019-20
Chapter 5 • Sensory, Attentional and Perceptual Processes
87
value of  those stimuli. The smallest difference
in the value of two stimuli that is necessary to
notice them as different is called difference
threshold or difference limen (DL). To
understand it, we may continue with our
“sugar water” experiment. As we have seen,
the plain water is experienced as sweet after
the addition of certain number of sugar
granules. Let us remember this sweetness. The
next question is: how many sugar granules
will be needed in the water in order to
experience its sweetness as different from the
previous sweetness. Go on adding sugar
granules one after another tasting the water
each time. After addition of a few granules,
you will notice at a point that the water is now
sweeter than the previous one. The number
of sugar granules added to the water to
generate an experience of sweetness that is
different from the previous sweetness on 50
per cent of the occasions will be called the DL
of sweetness. Thus, difference threshold is the
minimum amount of change in a physical
stimulus that is capable of producing a
sensation difference on 50 per cent of the
trials.
You may realise by now that understanding
of sensations is not possible without
understanding the AL and DL of different types
of stimuli (for example, visual, auditory), but
that is not enough. Sensory processes do not
depend only on the stimulus characteristics.
Sense organs and the neural pathways
connecting them to various brain centers also
play a vital role in this process. A sense organ
receives the stimulus and encodes it as an
electrical impulse. For being noticed this
electrical impulse must reach the higher brain
centers. Any structural or functional defect or
damage in the receptor organ, its neural
pathway, or the concerned brain area may lead
to a partial or complete loss of sensation.
Visual Sensation
Among all sense modalities, vision is the most
highly developed in human beings. Various
estimates indicate that we use it in
approximately 80 per cent of our transactions
with the external world. Audition and other
senses also contribute significantly to
information gathering from the external world.
We shall discuss vision and audition in some
detail. The main features of other senses can
be found in Box 5.1.
Visual sensation starts when light enters
the eyes and stimulates our visual receptors.
Our eyes are sensitive to a spectrum of light,
the wavelength of which ranges from 380 nm
to 780 nm (nm refers to nanometer, which is
one billionth of a meter). No sensation is
registered beyond this range of light.
The Human Eye
A diagram of the human eye is shown in
Figure 5.1. As you can see, our eye is made
up of three layers. In the outer layer, there is
a transparent cornea and a tough sclera that
surrounds the rest of the eye. It protects the
eye and maintains its shape. The middle layer
is called choroid, which is richly supplied with
blood vessels. The inner layer is known as
retina. It contains the photoreceptors (rods
and cones) and an elaborate network of
interconnecting neurons.
The eye is generally compared with a
camera. For example, the eye and camera have
a lens. The lens divides the eye into two
unequal chambers, namely aqueous chamber
and vitreous chamber. The aqueous chamber
is located between the cornea and the lens. It
is smaller in size and is filled with a water-
like substance, called aqueous humor. The
vitreous chamber is located between the lens
and the retina. It is filled with a jelly like
protein, called vitreous humor. These fluids
help in holding the lens at its appropriate place
and in proper shape. They also allow enough
flexibility for the occurrence of accommodation
— a process through which the lens changes
its shape in order to focus the objects at
varying distances. This process is regulated
by ciliary muscles, which are attached to the
lens. These muscles flatten the lens to focus
the distant objects and thicken it to focus the
near objects. Like a camera, the eye also has
a mechanism to control the amount of light
2019-20
Psychology
88
(colour) vision. Each eye contains about 100
million rods and about 7 million cones. The
cones are highly concentrated in the central
region of the retina surrounding the fovea,
which is a small circular region of the size of a
pea. It is also known as the yellow spot. It is
the region of maximum visual acuity. Besides
photoreceptors, retina also contains a bundle
of axons of a cell (called ganglion cell) that
forms the optic nerve, which leads to the
brain.
Working of the Eye
Passing through conjunctiva, cornea, and
pupil, the light enters the lens, which focuses
entering into it. The iris serves this purpose.
It is a disc-like coloured membrane lying
between the cornea and the lens. It controls
the amount of light entering the eye by
regulating pupil dilation. In dim light the pupil
dilates; in bright light it contracts.
Retina is the inner most layer of an eye. It
is made up of five types of photosensitive cells
among which rods and cones are most
important. Rods are the receptors for scotopic
vision (night vision). They operate at low
intensities of light, and lead to achromatic
(colourless) vision. Cones are the receptors for
photopic (day light) vision. They operate at high
levels of illumination, and lead to chromatic
Besides vision and audition, there are other
senses that enrich our perceptions. For example,
an orange looks attractive not only because of it
is colour but also because it has got a special
flavour and taste. These other senses are briefly
described here.
 1. Smell : The stimulus for smell sensation
consists of molecules of various substances
contained in the air. They enter the nasal
passage where they dissolve in moist nasal
tissues. This brings them in contact with
receptor cells contained in olfactory
epithelium. Human beings possess about 50
million of these receptors, whereas dogs
possess more than 200 million of these
receptors. Nevertheless, our ability to detect
smell is impressive. It is indicated that human
beings can recognise and distinguish among
approximately 10,000 different odours. The
sense of smell also shows sensory
adaptation like other senses.
2. Taste : The sensory receptors for taste are
located inside small bumps on the tongue,
known as papillae. In each papilla there is a
cluster of taste buds. Human beings possess
almost 10,000 taste buds. While people claim
to distinguish a large number of flavours in
food, there are only four basic tastes, namely
sweet, sour, bitter and salty. How is it then
that we perceive many more? The answer is
that we are aware not only of the taste of
the food, but also of its smell, its texture, its
Box Box Box Box Box 5.1 Other Human Senses Other Human Senses Other Human Senses Other Human Senses Other Human Senses
temperature, its pressure on our tongue, and
many other sensations. When these factors are
removed, we are left with only four basic tastes.
Besides, the combination of different flavours in
varied proportions results in a different kind of
flavour which may be quite unique.
3. Touch and other skin senses : Skin is a sensory
organ that produces sensations of touch
(pressure), warmth, cold, and pain. In our skin
there are specialised receptors for each one of
these sensations. The receptors of touch are not
evenly distributed in our skin. That is why some
areas of our body (e.g., face, fingertips) are more
sensitive than others (e.g., legs). Pain sensation
has no specific stimulus. Hence, determining its
mechanism has been fairly difficult.
4. The Kinesthetic system : Its receptors are found
primarily in joints, ligaments, and muscles. This
system gives us information about the location
of our body parts in relation to each other, and
allows us to perform simple (e.g., touching one’s
nose) and complex movements (e.g., dancing). Our
visual system provides a great deal of help in
this respect.
5. The Vestibular system : This system gives us
information about our body position, movement,
and acceleration — the factors that are critical
for maintaining our sense of balance. The sensory
organs of this sense are located in the inner ear.
While vestibular sacs inform us of our body
positions, the semicircular canals inform us about
our movements and acceleration.
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